Motorized door operators are known which include an electric motor driving a final output shaft by way of a belt or chain drive and intermediate shafts interposed the motor output shaft and the final output shaft. Depending on the type of door to be driven by the operator and the location of the operator, a relatively complex speed reduction drive train may be interposed an intermediate shaft and the final output shaft.
In all events, once a motor driven door operator of the general type discussed herein is mounted in its working position, it is often difficult to gain access to the operator for servicing or for replacement of wear items, such as endless drive belts. With conventional door operators, the motor support structure or an intermediate shaft is adjustable to adjust tension for and to replace endless drive belts. This adjustment or replacement activity is difficult to carry out when door operators are mounted high above the floor and not near any support structure which can be relied on to provide access by servicing personnel. Moreover, belt tension adjustment is subject to error which can cause premature belt and/or bearing failure. Accordingly, there has been a need to provide a door operator wherein the motor support means is not required to be moved nor is there any requirement to move an intermediate shaft driven by the motor output shaft by way of an endless belt. However, an improved overall drive mechanism arrangement, together with the provision of a stretchable endless drive belt in the mechanism drive train with belt drive pulleys located at fixed drive shaft centers, is provided by the present invention.